The sensation of pain can be categorized into two types, peripheral and central pain. Peripheral pain can be classified into three broad areas, nociceptive pain, inflammatory pain and neuropathic pain. Nociceptive pain is also referred to as physiological pain and serves as a defense mechanism throughout the animal kingdom. Inflammatory pain, arising from severe wounds and/or associated with inflammatory infiltrates, can be well controlled by non-steroidal anti-inflammatory drugs (NSAID)-like drugs, steroids and opiates. However, the etiology and management of neuropathic pain is not well understood. Neuropathic pain is thought to arise from inherent defects in sensory and as a consequence in sympathetic neurons and can be secondary to trauma.
Peripheral pain is mediated by two types of primary sensory neuron classes, the Ad- and C-fibers, whose cell bodies lie within the dorsal root ganglion. Although the mechanisms of generation of neuropathic pain are poorly understood it is clear that several factors influence the perception and transmission of the painful stimulus, namely, alterations in chemical environment, ectopic generation of sensory neuron firing and sympathetic discharge. Some of the most common syndromes associated with neuropathic pain arise from destruction of small sensory fibers (or possibly the alteration in ratios of small to large fibers) as it is common in post-traumatic situations. Other etiologies of pain arise from small fiber damage due to diabetic neuropathy, drug induced damage (chemotherapy drugs), alcoholism, damage due to cancer, and a variety of hereditary small- and large-fiber neuropathies. We rationalize that targets derived from the peripheral nervous system may be of strategic benefit in that candidate compounds do not need to cross the blood-brain barrier, they can act on the initiation site of pain without inducing central side effects.
It has long been established that central mechanisms are involved in the perception and modulation of pain. Electrical stimulation of the periaqueductal gray (PAG) area produces analgesia without loss of other sensory modalities. Descending pain pathways emanating from PAG and the nucleus raphe magnus impinge on dorsal spinal cord regions where primary nociceptive afferents terminate. Also, stimulation of regions such as the paragigantocellularis nucleus in the medulla oblongata result in analgesia. Finally, opiate receptors, when stimulated by opioid alkaloids and opioid peptides, mediate analgesia and these sites are located in key “pain centers” within the brain including PAG, thalamic nuclei and cortical regions. Identification of genes in these central nervous system (CNS) regions and the spinal thalamic tract from animal models of pain may elucidate important targets for pain modulation.